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Dye penetrant testing is ideal for surface-breaking defects in welds because it directly reveals cracks that open to the surface. The surface is cleaned, then a liquid dye is applied and allowed to seep into any openings by capillary action. After removing the excess dye, a developer is applied; the dye trapped in the crack is drawn out by the developer and becomes visible as a bright line, often under UV light if fluorescent dye is used. This makes even very small surface cracks easy to see and it works on a wide range of materials, is quick to perform, and is cost-effective.

Other methods have different strengths: magnetic particle inspection requires ferromagnetic materials and looks at surface and near-surface flaws; ultrasonic testing and radiography primarily detect internal flaws and require more equipment and setup. For detecting surface cracks in welds across many materials, dye penetrant testing is the practical choice.